gnu: pcmanfm-qt: Fix default wallpaper load path.

* gnu/packages/lxqt.scm (pcmanfm-qt)[arguments]: Patch "settings.ini" file and
update "LXQT_SHARE_DIR" environment variable to point to system profile,
instead of an invalid path pointing to "lxqt-build-tools" package.

Signed-off-by: Oleg Pykhalov <go.wigust@gmail.com>